1. Field of the Invention
The present invention relates to a method of immobilizing heavy metal contaminants in soil, and particularly to a method of reducing the amount of leachable lead in contaminated soil to non-hazardous levels.
2. Brief Description of the Prior Art
The presence of heavy metal contaminants, such as lead, in soil poses a major environmental hazard, particularly when the lead is capable of leaching into ground water supplies as a result of rain or other influx of water at the contaminated site. Acid rain only complicates this problem by increasing the solubility, and hence mobility, of lead contaminants.
Due to a growing concern over heavy metal contaminants in the environment, the Environmental Protection Agency has adopted strict guidelines for the amount of leachable lead that can be extracted from soil samples. The Resource Conservation Recovery Act (RCRA) regulations require that wastes containing lead, chromium, cadmium, arsenic, mercury, selenium, silver or barium leach minimum concentrations of these metals to be legally defined as non-hazardous.
Currently, solid wastes are tested using the EP (Extraction Procedure) Toxicity Test contained in 40 C.F.R. Chapter 1, Part 261, Appendix II (1987), the disclosure of which is hereby incorporated by reference. The EP Toxicity Test determines whether a solid waste has unacceptable levels of hazardous substances which can be leached by infiltrating water. The test is designed to simulate a worst-case leaching situation. A liquid extract is prepared from a solid waste sample, and is analyzed to determine whether the waste is "EP Toxic." A new test, the Toxic Characteristics Leaching Procedure (TCLP) has been developed and proposed by the Environmental Protection Agency (EPA), but has not yet been adopted.
Industry efforts to date directed to reducing the mobility of heavy metals in wastes have generally focussed on pretreatment of the wastes before final disposal. For example, existing processes to reduce chemical mobility in liquid or semi-solid wastes involve mostly encapsulation processes, whereby various encapsulants are added to the waste to turn the waste into a solid. Examples of such encapsulants are cement and sodium silicate, asphalt, glass, and various polymers and chemicals. These prior art encapsulation techniques attempt to isolate the contaminant by creating a hard solid mass which water cannot penetrate. Encapsulation processes are frequently used as final wastewater treatment steps to render sludges and liquids non-leachable. Unfortunately, encapsulation processes tend to be expensive and often greatly increase the volume of material requiring final disposal. Moreover, the encapsulated hazardous waste may be subject to leaching if the encapsulation seal subsequently fails.
Another prior art method for treating an aqueous waste containing heavy metals, such as lead, is disclosed by DOUGLAS et al., in U.S. Pat. No. 4,671,882, wherein a non-hazardous sludge is said to be produced by (a) adding phosphoric acid or an acid phosphate salt to the aqueous solution to precipitate the heavy metals; (b) lowering the pH of the solution to less than about 5.0; (c) adding a coagulant to the solution; (d) raising the pH of the solution to about 7.0 by the addition of a calcium source; and (e) dewatering the resulting non-hazardous sludge. Phosphoric acid is said to be preferred over acid phosphate salts (mono- or dihydrogen ammonium phosphate). Treatment with phosphoric acid or an acid phosphate salt precipitates the heavy metal and forms metal phosphates which are said to be much more insoluble than the corresponding metal hydroxides or sulfates.
Various attempts have also been made to pretreat solid wastes containing heavy metal contaminants. For example, O'HARA et al., U.S. Pat. No. 4,737,356, discloses a process for immobilizing lead and cadmium in solid wastes which involves the addition of a water-soluble phosphate, particularly phosphoric acid. The solids treated in accordance with the O'HARA et al. '356 patent include solid residues resulting from the burning of solid wastes. These solids are generally said to consist of fly ash, in whole or in part, since lead and cadmium tend to be concentrated in fly ash. The amount of water-soluble phosphate employed is said to depend on the alkalinity of the solid residue, its buffering capacity, and the amounts of lead and cadmium present. In general, from about 1 to 8% by weight of phosphoric acid is employed, together with a calcium hydroxide-providing composition such as lime, hydrated lime or flue gas scrubber product. O'HARA et al. teach that calcium phosphate, Ca.sub.3 (PO.sub.4).sub.2, is inoperative in immobilizing lead and cadmium. See column 3, lines 3-6 and Example 11 of O'HARA et al. '356.
O'HARA et al., U.S. Pat. No. 4,629,509, is directed to a process in which cadmium and lead are immobilized in a dry solid residue, i.e. fly ash, by mixing equimolar proportions of lime and an aqueous solution of a soluble inorganic salt supplying sulfide. The use of calcium sulfide is said to eliminate the necessity of adding lime in order to immobilize the cadmium or lead contaminant.
STANFORTH, U.S. Pat. No. 4,950,409, is directed to a method of treating solid hazardous wastes containing unacceptable levels of leachable metals such as lead and cadmium. The solid waste is mixed with lime and an agent selected from carbon dioxide and bicarbonate. Reaction between the metals and agent is said to convert the metals to non-leachable forms which are relatively stable under normal environmental conditions.
Similarly, STANFORTH, U.S. Pat. No. 4,889,640, discloses a method of treating solid hazardous wastes containing unacceptable levels of leachable metals such as lead, in which the solid waste is mixed with a water treatment lime sludge which includes an agent selected from calcium carbonate, magnesium carbonate and calcium magnesium carbonate. The agent and the metal are said to convert the metal to a non-leachable form which is relatively stable under normal conditions.
The above-described efforts, while providing potential means for pretreating concentrated hazardous wastes prior to ultimate disposal, fail to provide a viable method of treating existing hazardous waste sites that contain large volumes of soil contaminated with untreated heavy metals.
It would therefore be highly desirable to provide a method for effectively immobilizing heavy metal pollutants such as lead in soil located at a hazardous waste site. Such a treatment technique would ideally be suitable for application to large volumes of earthen material, utilize economical and environmentally safe treating agents, and be adapted for implementation outdoors.